A fuse is a circuit element that is designed to self destruct when too much current flows through the fuse. The destruction of the fuse breaks the circuit, stops the flow of current and protects other circuit elements from being damaged. In semiconductor integrated circuits a laser trim technique is used to apply laser energy to blow a fuse.
Laser trimmed fuses typically comprise a strip of aluminum metal. The strip of aluminum metal is the metal link layer that is broken when the fuse is blown. Aluminum has a relatively low melting point and a high surface tension. When the aluminum metal link layer receives laser energy the energy creates heat that melts the aluminum. The melted aluminum then vaporizes and breaks the electrical circuit through the fuse. As the melted aluminum vaporizes the melted aluminum tends to splatter.
To prevent retention of traces of metal and link splattering and to prevent standing wave effects, the metal link layer of the fuse is covered with a thin layer of oxide. The thin layer of oxide enables the melted aluminum metal to build up pressure until it explosively vaporizes through the oxide layer. The thin layer of oxide also separates the metal link layer from an upper layer of metal that contains pad connections that are accessible from outside the integrated circuit.
It is very important that the thin layer of oxide over the metal link layer be uniform in thickness. If there is no oxide at all over the metal link layer, then the metal link layer will be subject to corrosion. If the thin layer of oxide is too thin (e.g., less than two thousand Ångstroms), then cracks in the fuse may form that are likely to cause yield loss and reliability concerns. If the thin layer of oxide is too thick (e.g., more than eight thousand Ångstroms), then a high laser energy will be required to blow the fuse. The use of a high laser energy can easily lead to substrate damage, again causing yield loss and reliability concerns.
An optimum thickness for the thin layer of oxide is approximately one half of the laser wavelength. The laser wavelength is typically about one micron. A micron is equal to one millionth of a meter (10−6 m). An Ångstrom is equal to one ten thousandth of a micron (10−10 m). Therefore the optimum thickness for the thin layer of oxide is about five thousand Ångstroms (5000 Å).
One prior art method for creating the thin layer of oxide over the metal link layer of the fuse employs a masked partial etch-back of the passivation layers over the final layer of metal. Partial etch-back processes are difficult to control due to variations from machine to machine and due to variations over time in both deposition thicknesses and etch rates. Prior art etch-back processes are often done with an endpoint detection technique that monitors the plasma for changes in emission and/or wavelength distribution.
In laser trimming the endpoint detection method can not be used (1) because the film in question is still being etched in the entire process, and (2) because the trim structures occupy only a small percentage of the die.
Therefore, there is a need in the art for a system and method that is capable of providing a uniform oxide layer over a metal link layer of a laser trimmed fuse. In particular, there is a need in the art for a system and method that is capable of providing an oxide layer over a metal link layer of a laser trimmed fuse that is approximately five thousand Ångstroms thick.